1. Field of the Invention
This invention relates to an aqueous emulsion comprising a self-emulsifiable copolymer which is rich in flexibility, tough and excellent in durability; a process for producing the same; and an aqueous paint and aqueous printing ink comprising the same.
2. Related prior art
Organic solvent-containing paints and printing inks have problems with safety and hygiene such as an unhealthy effect by inhalation of organic solvent vapors, explosion, fire and the like, and environmental problems such as air pollution and the like, so that the development of aqueous systems has recently been actively conducted.
In an aqueous paint, there has heretofore been used aqueous acrylic emulsions which are good in weather resistance and gloss. However, aqueous acrylic emulsions are disadvantageous in that the adhesion to a substrate and the flexibility of coating film are inferior. For overcoming these disadvantages, an attempt has been made to introduce a polyurethane skeleton excellent in elasticity and adhesiveness into the acrylic component.
For example, JP-A-59-138,210 proposes a method for polymerizing a vinylic monomer in an aqueous polyurethane.
JP-B-3-79,392 proposes a method for blending a polyether urethane with an aqueous acrylic resin.
Japanese Patent 2,641,955 proposes an aqueous resin composition obtained by polymerizing a water-soluble monomer having an .alpha.,.beta.-unsaturated double bond using a thiol group-containing thiourethane resin as a chain transfer agent and then subjecting the resulting polymer to salt-formation or quaternization with an alkali to make the polymer aqueous.
JP-A-5-339,542 proposes a method in which an anionic, aqueous polyurethane emulsion in which a hydrazide compound is used as a neutralizing agent is blended with a keto group-containing acrylic emulsion and the resulting blend is subjected to cross-linking reaction during the formation of a coating film.
JP-A-7-228,829 proposes a method in which an aqueous polyurethane emulsion containing a hydrazide group is blended with a keto group-containing acrylic emulsion and the resulting blend is subjected to cross-linking reaction during the formation of a coating film.
JP-A-7-233,347 proposes an aqueous binder which is self-cross-linkable and composed of a polyhydrazide and a carbonyl group-containing urethane-vinyl hybrid polymer. This urethane-vinyl hybrid polymer is produced by subjecting to free radical-initiated polymerization an ionic and/or non-ionic, stabilized polyurethane macromonomer having terminal and/or side vinyl groups and other functional and non-functional vinyl monomers having carbonyl groups.
JP-A-8-165,318 proposes a method for copolymerizing an aqueous polyurethane having a polymerizable, unsaturated group with a vinylic monomer.
JP-A-9-25,324 discloses a method for producing an acryl-urethane copolymer in which a urethane prepolymer having a mercapto group is used as a chain transfer agent.
However, each of the above methods has the following disadvantages:
In the methods described in JP-A-59-138,210 and JP-B-3-79,392, the acrylic emulsion and the polyurethane emulsion are merely physically mixed and not bonded with each other. Acrylic resin and urethane resin are have poor compatibility, so that it follows that haze is caused on the coating film obtained (haze increases).
In the aqueous resin composition described in Japanese Patent 2,641,955, a hydrophilic, polar group is present in only the acrylic segment and is not introduced into the thiourethane portion. Therefore, in order to allow the whole resin to have a water-dispersibility, many hydrophilic, polar groups must be introduced. If so, the coating film formed therefrom becomes insufficient in water resistance.
According to the method described in JP-A-5 25 339,542, the acrylic component and the urethane component in the coating film are bonded through the hydrazide compound which is the neutralizing agent. However, in the emulsion state, the urethane resin and the acrylic resin are merely blended and since the urethane resin and the acrylic resin have poor compatibility, it follows that haze is caused on the coating film obtained (haze increases).
According to the method described in JP-A-7 228,829, in the coating film the urethane resin and the acrylic resin are bonded by the reaction between the hydrazide group in the urethane resin and the carbonyl group in the acrylic resin. However, in the emulsion state, the urethane resin is merely blended with the acrylic resin, and since the urethane resin and the acrylic resin have poor compatibility, it follows that haze is caused on the coating film obtained (haze increases).
In the aqueous binder described in JP-A-7-233,347 and the method described in JP-A-8-165,318, no chain transfer agent is used in the course of the resin production and hence a polyurethane resin having at least two ethylenically unsaturated double bonds in one molecule is produced, so that when it is copolymerized with a vinylic monomer, gelation occurs. Therefore, in the formation of a coating film, fusion between particles is inhibited and the appearance of coating film is poor.
The acryl-urethane copolymer described in JP-A-9-25,324 cannot be applied to an aqueous system.
Moreover, in the conventional aqueous printing inks, there is none which adheres to both polar substrates such as polyethylene terephthalate (PET) or nylon (NY) and non-polar substrates, for example, a polyolefin such as polyethylene (PE), polypropylene (PP) or the like.
In order to reduce the above problems, as in JP-A-6-80,930, JP-A-10-139,839 and the like, blending the acrylic resin with the urethane resin and copolymerization of the starting monomers of these resins have been examined.
However, according to the method described in JP-A-6-80,930, an acrylic resin is produced in a polyurethane emulsion, and hence, fundamentally, the urethane resin and the acrylic resin are only mixed. The acrylic resin and the urethane resin have poor compatibility, and hence the adhesiveness of a print film after water has been driven off tends to become inferior. Moreover, in the method described in JP-A-10139,839, no chain transfer agent is used in the course of the resin production and hence a polyurethane resin having at least two ethylenically unsaturated double bonds in one molecule is produced, so that when it is copolymerized with a vinylic monomer, gelation tends to occur.
An object of this invention is to provide an aqueous emulsion containing a self-emulsifiable copolymer which can form a coating film rich in flexibility, tough, excellent in durability and low in haze.
Another object of this invention is to provide a process for producing the above aqueous emulsion.
A still another object of this invention is to provide an aqueous paint in which the above aqueous emulsion is used.
A further object of this invention is to provide an aqueous printing ink excellent in adhesiveness to all substrates including polyolefin substrates, pigment dispersibility and redissolvability.
Other objects and advantages of this invention will become apparent from the following description.